1. Field of the Invention
The present invention relates to a multi-speed sprocket assembly for a bicycle, and more particularly to a multi-speed sprocket assembly of the above type including at least one large sprocket with teeth and one small sprocket with teeth disposed adjacent each other and selectively engageable with a drive chain of the bicycle consisting of a looped series of pivotably connected link plates.
2. Description of the Prior Art
A typical multi-speed sprocket assembly of the above-described type is known from a Japanese published utility model gazette No. 55-28617. In this sprocket assembly, in order to provide good chain engaging performance, the large sprocket and the small sprocket disposed adjacent thereto are arranged so that a central point between an adjacent pair of teeth of the large sprocket is positioned on a tangential line extending from a central point between an adjacent pair of teeth of the small sprocket with a distance between the central points being an integer multiple of a pitch of the driver chain.
For effecting a change-speed operation by shifting the drive chain from one sprocket to another, a portion of the chain postioned on an upstream side of the sprocket assembly relative to a rotational drive direction of the same is pushed by a derailleur device in a rotational axis direction of the sprocket assembly. With the above conventional construction, if the small sprocket is significantly smaller in diameter than the large sprocket, when a chain portion positioned more upstream than a chain portion currently engaging with the small sprocket is shifted toward the large sprocket, most of this chain portion moving toward the large sprocket is already disengaged from the teeth of the small sprocket, whereby the chain shifting operation can take place smoothly without being interferred by trailing engagement with the small sprocket.
However, in the case of the above conventional construction, if the diameter difference between the small sprocket and the large sprocket is rather small, namely, if the difference in the numbers of gears between these sprockets is small, most of the leading chain portion, i.e. the chain portion to be first shifted onto the large sprocket is still engaged with the teeth of the small sprocket, such that the remaining engagement with the small sprocket tends to interfere with the chain shifting movement to the large sprocket. Accordingly, the leading chain portion first rides onto a tooth edge of the small sprocket and then comes into engagement with a leading tooth of the large sprocket.
Taking the above-described state of the art into consideration, the primary object of the present invention is to provide a multi-speed sprocket assembly for a bicycle which permits smooth change speed operation by shifting a drive chain from a small sprocket to a large sprocket even if a diameter difference between the small and large sprockets is rather small.